Fiber stretcher module for use in the 1550 nm wavelength range

ABSTRACT

Embodiments of the present invention are generally related to embodiments of the present invention relate to a fiber stretchers module for use in the 1550 nm wavelength range. In one embodiment of the present invention, a fiber stretcher module for use in the 1550 nm wavelength range comprises a fiber having a relative dispersion slope, RDS, and a relative dispersion curvature, RDC, wherein a ratio of said slope to said curvature is between about 30 nm and about 0 nm, having a dispersion value of less than about −10 ps/(nm·km) at about 1550 nm, and a RDS is equal to or greater than 0.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of co-pending patent application Ser. No. 14/126,058, filed on Dec. 13, 2013, entitled “FIBER STRETCHER MODULE FOR USE IN THE 1550 NM WAVELENGTH RANGE”, which is owned by the assignee of the present application, and which is incorporated herein by reference in its entirety. The patent application Ser. No. 14/126,058 claims the priority benefit of Provisional Patent Application Ser. No. 61/503,044, filed Jun. 30, 2011, which is owned by the assignee of the present application, and which application is incorporated herein by reference.

BACKGROUND

1. Field of the Invention

Embodiments of the present invention are generally related to embodiments of the present invention relate to fiber stretcher modules for use in the 1550 nm wavelength range. More specifically, embodiments of the present invention relate to a fiber stretcher module for dispersion management in short pulse laser systems.

2. Description of the Related Art

A fiber stretcher module offers a solution for known femtosecond fiber based chirped pulse amplification systems that typically use bulk optics or a chirped fiber Bragg grating to stretch the pulses. A chirped fiber Bragg grating generally yields large group delay ripples as well as significant amplitude ripples which are highly undesirable. As such, fiber-based stretchers are preferable over stretchers made of bulk optics due to smaller size, better stability, longer lifetime, lower cost and improved characteristics over the drawbacks described above.

A difficulty with using a fiber stretcher is that it must match a particular compressor grating (i.e., it must have an equal quantitative dispersion with an opposite sign—that is, positive/negative). Generally, the compressor grating has high numerical values for both third order dispersion β₃ and fourth order dispersion β₄. For example, a 1200 lines/mm compressor grating working at an incidence angle of 64.5° with a second order dispersion of β₂=−57.4 ps² at 1550 nm should have the below dispersion values: β₂=−57.4 ps²; β₃=3.17 ps³; and β₄=−0.291 ps⁴

At 1550 nm, these values correspond to:

D=45 ps/nm; RDS=0.0420 nm⁻¹; and RDC=0.00295 nm⁻²

Where D is the dispersion, RDS is the relative dispersion slope, and RDC is the relative dispersion curvature of the grating.

However, using a known stretcher module made of a combination of an existing Dispersion Compensation Fiber (DCF) and Super Large Effective Area (SLA) fiber, i.e., using around 0.90 km of Ultra-High Slope (UHS) DCF and 3.6 km of SLA, a stretcher module with the below dispersion values at 1550 nm can be made:

β₂=57.2 ps²; β₃=−3.17 ps³; and β₄=0.14 ps⁴

While it is observed that β₂ and β₃ are very close to target, β₄ is ˜50% of matching that of the desired grating. As such, a new solution is needed achieve the desired match between the stretcher module and a chosen grating.

SUMMARY

Embodiments of the present invention are generally related to embodiments of the present invention relate to fiber stretcher modules for use in the 1550 nm wavelength range. More specifically, embodiments of the present invention relate to a fiber stretcher module for dispersion management in short pulse laser systems.

In one embodiment of the present invention, a fiber stretcher module for use in the 1550 nm wavelength range comprises a first fiber having a ratio of relative dispersion slope (RDS) over relative dispersion curve (RDC) value of less than about 30 nm and a dispersion value, D, of less than about −10 ps/(nm·km) at about 1550 nm, and a second fiber having a RDC value of about zero, a relative RDS value greater than 0.002 nm⁻¹, and a dispersion, D, value of greater than about 10 ps/(nm·km) at about 1550 nm, wherein the lengths of the first and second fibers are adjusted to obtain a best possible dispersion match to a corresponding compressor grating, or other types of compression devices, operating at a wavelength range of Er-doped fiber amplifiers and lasers, which may generally be between about 1520 nm to about 1620 nm (i.e., the 1550 nm range).

In another embodiment of the present invention, a chirped pulse amplification system comprises a pulse generator producing pulses in the 1550 nm wavelength range, and a fiber stretcher module for use in this range, an amplifying element, and a compressor grating or other compression device, where the fiber stretcher module comprises: a first fiber comprising a ratio of relative dispersion slope over relative dispersion curvature (RDS/RDC) value of less than about 30 nm and a dispersion, D, value of less than about −10 ps/(nm·km) at about 1550 nm, and a second fiber comprising a RDC value of about zero, a RDS value greater than 0.002 nm⁻¹, and a dispersion value of greater than about 10 ps/(nm·km) at about 1550 nm, wherein the lengths of first and second fibers are adjusted to get the best possible dispersion match to a corresponding compressor grating at about 1550 nm.

In yet another embodiment of the present invention, a method of operating a chirped pulse amplification system in the 1550 nm wavelength range, comprising: providing a pulse generator producing pulses in the 1550 nm wavelength range, providing a fiber stretcher module for use in the 1550 nm wavelength range, a amplifier stage, and a compressor grating, where the fiber stretcher module for use in the 1550 nm wavelength range comprise: a first fiber comprising a ratio of relative dispersion curvature over relative dispersion slope (RDS/RDC) value of less than about 30 nm and a dispersion value of less than about −10 ps/(nm·km) at about 1550 nm, and a second fiber comprising a relative dispersion curvature value of about zero and a relative dispersion slope value greater than 0.002 nm⁻¹ and a dispersion value of greater than about 10 ps/(nm·km) at about 1550 nm, wherein the length of first and second fiber is adjusted to get the best possible dispersion match to a corresponding compressor grating or other compression device, at about 1550 nm.

BRIEF DESCRIPTION OF THE DRAWINGS

So the manner in which the above-recited features of the present invention can be understood in detail, a more particular description of embodiments of the present invention, briefly summarized above, may be had by reference to embodiments, which are illustrated in the appended drawings. It is to be noted, however, the appended drawings illustrate only typical embodiments of embodiments encompassed within the scope of the present invention, and, therefore, are not to be considered limiting, for the present invention may admit to other equally effective embodiments, wherein:

FIG. 1 is a graph of a RDS vs. RDC target curves and results of a stretcher module comprising a known fiber and an exemplary fiber designed in accordance with embodiments of the present invention;

FIG. 2 is a graph showing refractive index profiles of a stretcher module comprising a known fiber and an exemplary fiber designed in accordance with embodiments of the present invention;

FIG. 3 is a graph showing modeled properties for dispersion and RDS values for a known fiber in accordance with embodiments of the present invention;

FIG. 4 is a graph showing modeled properties for dispersion and RDS values for one exemplary fiber design in accordance with embodiments of the present invention;

FIG. 5 is a graph showing modeled RDC properties for a known fiber in accordance with embodiments of the present invention;

FIG. 6 is a graph showing modeled RDC properties for an exemplary fiber designed in accordance with embodiments of the present invention;

FIG. 7 is a graph showing modeled effective index differences relative to the cladding for a known fiber in accordance with embodiments of the present invention;

FIG. 8 is a graph showing modeled effective indexes differences relative to the cladding for an exemplary fiber designed in accordance with embodiments of the present invention;

FIG. 9 is a graph showing modeled bend loss of an exemplary fiber designed in accordance with embodiments of the present invention;

FIG. 10 is a graph showing an index profile of an exemplary fiber designed in accordance with embodiments of the present invention and examples of two realizations, i.e., two different preforms with different index profiles;

FIG. 11 is a graph showing measured RDC versus RDS values for two exemplary fibers designed in accordance with embodiments of the present invention as compared to a target; and

FIG. 12 is a graph showing the results of a special cut off measurement analysis performed on an exemplary fiber designed in accordance with embodiments of the present invention.

The headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description or the claims. As used throughout this application, the word “may” is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). Similarly, the words “include”, “including”, and “includes” mean including but not limited to. To facilitate understanding, like reference numerals have been used, where possible, to designate like elements common to the figures.

DETAILED DESCRIPTION

Embodiments of the present invention are generally related to embodiments of the present invention relate to fiber stretcher modules for use in the 1550 nm wavelength range. More specifically, embodiments of the present invention relate to a fiber stretcher module offering an alternative solution for femtosecond fiber-based chirped pulse amplification systems without using bulk optics or chirped fiber Bragg gratings to stretch the pulses.

As used herein, the term “about” or “approximately,” or derivatives thereof, when referencing a numerical value should be deemed to include within ten percent of such numerical value in either direction. In addition, when such terms are utilized to described absolutes (e.g., zero), the absolute should be deemed to include within one unit of reasonable measurement in either direction, as would ordinarily be used by those of ordinary skill in the art.

In many embodiments of the present invention, the fiber stretcher module may be suitable for any fiber optic system operating in the 1550 nm range. In one embodiment, the fiber stretcher module may be utilized with any chirped pulse amplification configuration operating in the 1550 nm wavelength range. In such an embodiment, a pulse generator may provide pulses to a system in the 1550 nm wavelength range.

In addition, in accordance with embodiments of the present invention, the fiber stretcher module is capable of single mode operation, despite its multi-moded design. Generally, it obtains such capability through high bend loss of higher order modes.

A fiber stretcher module made in accordance with embodiments of the present invention gives an advantage over current modules using bulk optics or chirped FBG by providing less group delay ripples as well as less amplitude ripples. In addition, the fiber stretcher module is more desirable due to smaller size, better stability, longer lifetime, and lower cost over other fiber solutions.

In one embodiment of the present invention, a fiber stretcher module comprises a first fiber and a second fiber, which, when combined, yields target β₂, β₃, and β₄ values of approximately 57.4 ps², −3.17 psi, and 0.291 ps⁴, respectively at 1550 nm. To obtain such values, in many embodiments, the fiber stretcher module has an increased RDC compared to currently known modules, e.g., the UHS-DCF module described above.

In accordance with embodiments of the present invention, the RDCc for a dispersion compensating fiber combined (DCFC) comprising the combination of two fibers, i.e., a first fiber (F1) and a second fiber (F2), can be calculated from:

${RDC}_{C} = {\frac{1}{{RDS}_{F\; 1} - {RDS}_{F\; 2}}\left( {{\left( {{RDS}_{C} - {RDS}_{F\; 2}} \right){RDC}_{F\; 1}} - {\left( {{RDS}_{C} - {RDS}_{F\; 1}} \right){RDC}_{F\; 2}}} \right)}$

In one embodiment, F2 is a SLA fiber with: RDS_(F2)=0.003 nm⁻¹, RDC_(F2)=0. In such an embodiment, by utilizing target RDS and RDC values of the combined module of RDS_(C)=0.0420 nm⁻¹ and RDC_(C)=0.00295 nm⁻², a desired or target ratio between RDC and RDS for F1 can be calculated. FIG. 1 depicts a graph of a RDC versus RDS target curve and experimental results of a known fiber (UHS) and an exemplary fiber (design 9) designed in accordance with embodiments of the present invention. FIG. 1 also shows lines illustrating fixed RDS/RDC ratios. It is observed that the target curve is close to the 15 nm RDS/RDC line of design 9, while the existing UHS fiber is characterized by a RDS/RDC ratio of 32 nm.

Based upon the above relationship, in the embodiment described, the first fiber (i.e., F1) may be designed to achieve the intended performance characteristics. In another embodiment, the first fiber, F1, may further comprise each of the following limitations: (1) the fiber has a micro-bend loss the same as or better than the current or known fiber designs (2) the fiber has a RDC value greater than 0.0002 nm⁻² at a wavelength of 1550 nm; and/or (3) the fiber has a dispersion at 1550 nm in the range of −40 to −70 ps/(nm·km). Such a combination of characteristics yields a fiber design suitable for embodiments of the present invention.

Based on the requirements above, simulation experiments were conducted using different fiber designs, and the results are presented herein.

FIG. 2 depicts a graph showing a desired refractive index profile of a fiber in accordance with embodiments of the present invention as compared to a known UHS fiber. Both fibers are characterized by having three principal features. A central core with a positive Δn relative to the outer cladding, surrounded by a trench with a negative Δn relative to the outer cladding, further surrounded by a ring with positive Δn relative to the outer cladding, followed by the outer cladding. In a physical realization of the fiber shown in FIG. 2, the core typically comprises silica doped with germanium, the trench comprises silica doped with fluorine and, optionally, a small amount of germanium, and the ring comprises silica doped with germanium and, optionally, a small amount of fluorine. The outer cladding may be made of pure silica. Between the ring and the outer cladding, there may be an inner cladding layer (with around the same Δn as the pure silica cladding) consisting of silica doped with fluorine and further doped with phosphor and/or germanium. It is noted that the refractive index profile of this fiber as compared to a known UHS fiber is distinguished primarily by having a broader trench and a ring with a higher Δn relative to the outer cladding.

FIGS. 3 and 4 depict graphs comparing the modeled properties for dispersion and RDS values for a known UHS fiber and for the design 9 fiber in accordance with embodiments of the present invention, respectively.

FIGS. 5 and 6 depicts graphs showing modeled properties for RDC values for a known UHS fiber, and for target design 9 fiber in accordance with embodiments of the present invention, respectively. As shown, the design 9 provides a significantly higher RDC value around the 1550 nm operating range. When comparing the dispersion, RDS, and RDC of the design 9 fiber to those of the UHS design, it is noted that the increased RDS/RDC ratio is obtained by decreasing the wavelength range of the negative dispersion peak, which simultaneously has become much deeper. This is a consequence of the broader trench and the higher Δn of the ring of design 9 compared to that of the UHS design.

FIGS. 7 and 8 depict modeled effective indices for the fundamental mode LP₀₁ as well as for the first three higher order modes LP₀₂, LP₁₁ and LP₂₁ for a known UHS fiber, and for the design 9 fiber, respectively. It is observed in FIG. 8, at 1550 nm in addition to LP₀₁, the LP₀₂, LP₁₁ and LP₂₁ modes seem to have a substantial effective index, which indicates the design 9 fiber is not single moded. However, as shown in FIG. 9, which depicts a graph showing bend loss for the design 9 fiber, the higher order modes of this fiber have substantially higher bend loss, thereby assuring single mode operation.

Two different preforms were manufactured and drawn into fibers. FIG. 10 depicts a graph showing examples of index profiles of two physical realizations in accordance with embodiments of the present invention as compared to the targeted design 9 fiber index profile. The results of such measured characteristics of the fibers drawn from the two preforms are shown below in Table 1 and Table 2 respectively.

TABLE 1 attenuation attenuation attenuation attenuation dispersion RDS RDC length @1310 nm @1410 nm @1550 nm @1625 nm @1550 nm @1550 nm @1550 nm PMD Preform id Spool id m dB/km dB/km dB/km dB/km ps/(nm · km) nm⁻¹ nm⁻² ps · km^(−0.5) 90519371 21000000 10019 0.975 0.888 0.591 1.934 −41.04 0.009988 0.00045424 90519371 22000000 10019 0.926 0.852 0.560 1.137 −34.6 0.006534 0.00029807 90519371 23000000 10019 0.928 0.859 0.564 0.921 −33.88 0.005485 0.00013972 90519371 24000000 10014 0.926 0.858 0.563 1.000 −34.5 0.005833 0.00027502 90519371 25000000 10014 0.940 0.864 0.568 1.463 −34.38 0.005618 0.00013382 90519371 26000000 10014 0.936 0.860 0.564 0.877 −33.95 0.004932 0.00017107 90519371 27000000 10014 0.930 0.860 0.563 0.801 −33.75 0.005324 0.00012471 0.032 90519371 28000000 10024 0.937 0.866 0.564 0.719 −33.57 0.004786 0.00022643 90519371 29100000 10019 0.928 0.863 0.560 1.244 −36.22 0.006683 0.00029546 90519371 29200000 10009 0.933 0.868 0.563 1.022 −36.2 0.006712 0.00021409 90519371 29400000 10019 0.933 0.869 0.563 1.396 −38.05 0.007591 0.00034645 90519371 29500000 10014 0.965 0.931 0.557 1.688 −40.71 0.009453 0.00041642 0.032 90519371 29600000 10014 0.932 0.867 0.560 1.639 −39.72 0.008826 0.00036873 90519371 29700000 10014 0.935 0.860 0.564 1.521 −40.03 0.00883 0.0003046 90519371 29800000 10009 0.959 0.916 0.560 1.601 −40.94 0.00932 0.00036143 0.048 90519371 29910000 10019 0.925 0.859 0.561 1.618 −41.12 0.009677 0.00039108 90519371 29920000 10009 0.922 0.862 0.563 1.714 90519371 29930000 10009 0.919 0.853 0.557 1.635 −43.3 0.011115 0.00050825 90519371 29940000 10019 0.924 0.850 0.554 1.616 −42.81 0.010879 0.00053815 0.056 90519371 29950000 4861 0.953 0.909 0.552 2.442 −43.21 0.011847 0.00055459 0.085 90519371 29960000 9288 0.984 0.934 0.575 2.135 −45.9 0.013275 0.00060773 0.056

TABLE 2 attenuation dispersion RDS length @1550 nm @1550 nm @1550 nm RDC @1550 nm PMD Preform id Spool id m dB/km ps/(nm · km) nm⁻¹ nm⁻² ps · km^(−0.5) 110224371 72000000 10019 0.5625 −46.14 0.013455 0.00072957 0.027 110224371 73000000 10009 0.5595 −43.25 0.011707 0.00060404 0.017 110224371 74000000 10009 0.5595 −42.95 0.010896 0.00054881 0.02 110224371 75000000 10009 0.556 −41.7 0.00961 0.00045269 0.038 110224371 76000000 10014 0.5525 −41.32 0.009198 0.0003235 0.135 110224371 77000000 10014 0.557 −43.71 0.011118 0.00053023 0.172 110224371 78000000 10014 0.555 −42.33 0.010222 0.0005101 0.142 110224371 79100000 10014 0.558 −45.9 0.012567 0.00064317 0.15 110224371 79200000 10014 0.5525 −44.87 0.011487 0.00055535 0.273 110224371 79300000 10009 0.5535 −49.73 0.014962 0.00078894 0.094 110224371 79400000 10014 0.5565 −51.33 0.014464 0.00062795 0.144 110224371 79500000 10009 0.557 −61.27 0.020728 0.00111174 0.23 110224371 79600000 10019 0.5595 −59.74 0.019709 0.00099303 0.027 110224371 79700000 10009 0.5645 −60.18 0.019488 0.00108797 0.049 110224371 79910000 10004 0.5515 −59.52 0.020206 0.00112428 0.135 110224371 79920000 5446 0.5475 −59.61 0.021246 0.0010569 0.065 110224371 79930000 10009 0.556 −60.4 0.020153 0.0010602 0.065 110224371 79940000 6247 0.57 −69.42 0.026332 0.0017611 0.083 110224371 79950000 5143 0.5715 −83.26 0.027574 0.00196319 0.089

FIG. 11 depicts a graph showing a relationship between measured RDC and RDS values for the experimental tests discussed above in Table 1 and 2. As shown, despite the large spread in experimental values, the designed fibers, Realized 1 and Realized 2 produce RDS/RDC ratio values falling close to the target line.

In addition to the experimental tests described above, to examine whether single mode operation of the fiber in long length is obtained, a special cut off measurement was performed. The experiment involved measuring the attenuation of the fiber due to a Ø40 mm mandrel after propagation through 10 km of fiber, spooled on a spool with a bobbin radius of 90 mm. FIG. 12 depicts a graph showing the results of the special cut off measurement analysis. As shown in the Figure, with the applied winding conditions, the fiber is single mode over about 1480 nm after 10 km.

In view of all the experimentation conducted, as described herein above, an exemplary fiber in accordance with embodiments of the present invention may be combined with an SLA fiber to create a stretcher fiber module in accordance with embodiments of the present invention. In accordance with another exemplary embodiment, a stretcher fiber module was made using a fiber as described hereinabove, more specifically, a fiber as described in Table 1 was combined with a known SLA fiber to create a stretcher fiber module having the properties displayed below in Table 3.

TABLE 3 Fiber ID: 0904 45070001 Dispersion @ 1550 nm ps/nm −45.2 Beta 2 @ 1550 nm ps² 57.7 Beta 3 @ 1550 nm ps³ −3.23 Beta 4 @ 1550 nm ps⁴ 0.25 Max Insersion loss 1540 to 1560 nm dB 6.0 PMD ps 0.17 MPI 1540 to 1560 nm dB <−41 Length total m 14621 Length DCF m 5459 Length SLA m 9162 Effective area @ 1550 nm DCF μm² 13 Effective area @ 1550 nm SLA μm² 112 Dispersion coefficient @ 1550 nm DCF ps/(nm · km) −42.0 Dispersion coefficient @ 1550 nm SLA ps/(nm · km) 20.2

Analyzing the data of Table 3, it is observed that when compared to the known UHS/SLA module, a much better match for the fourth order dispersion parameter, β4, is obtained. Although the experimental testing resulted in an insertion loss being a little high (i.e., 6.0 dB), such results are likely due to a long fiber length, and/or as a result of the lower RDS of the realized fiber as shown in Table 1 as compared to the target design (e.g., compare FIG. 11 with FIG. 1). In addition, it should be noted the multipath interference (MPI) measurement is quite low (i.e., below the detection limit of the setup), which generally confirms the module operates in a single mode.

It should be appreciated, through embodiments of the present invention, the design techniques utilized to design the fiber stretcher module, and in particular, the first fiber therein, may be utilized by those of ordinary skill in the art to develop equally effective fiber assemblies for related applications. As such, embodiments of the present invention should be deemed inclusive of such modifications, as understood by one of ordinary skill in the art.

While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. It is also understood that various embodiments described herein may be utilized in combination with any other embodiment described, without departing from the scope contained herein. In addition, embodiments of the present invention may be further scalable, as particular applications may require. 

What is claimed is:
 1. An optical fiber stretcher module for use in the 1550 nm wavelength range comprising: a fiber having a relative dispersion slope, RDS, and a relative dispersion curvature, RDC, wherein a ratio of said slope to said curvature is between about 30 nm and about 0 nm, having a dispersion value of less than about −10 ps/(nm·km) at about 1550 nm, and a RDS is equal to or greater than
 0. 2. The fiber stretcher module of claim 1, wherein the RDS of the fiber in the 1550 nm range is between about 0.004 nm⁻¹ to about 0.03 nm⁻¹.
 3. The optical fiber stretcher module of claim 1, wherein the RDC of the fiber in the 1550 nm range is between about 0.0001 nm⁻² to about 0.002 nm⁻².
 4. The optical fiber stretcher module of claim 1, wherein the dispersion of the fiber in the 1550 nm range is between about −20 ps/(nm·km) to about −100 ps/(nm·km).
 5. The optical fiber stretcher module of claim 1, further comprising: a second fiber having a RDC of about zero, a RDS value greater than 0.002 nm⁻¹ and a dispersion value of greater than about 10 ps/(nm·km), at about 1550 nm.
 6. The optical fiber stretcher module of claim 5, wherein the second fiber comprises a super large effective area fiber, having a dispersion value of about 20 ps/(nm·km) and a relative dispersion slope of about 0.003 nm⁻¹, at about 1550 nm.
 7. The optical fiber stretcher module of claim 5, wherein the optical fiber stretcher module in the 1550 nm range has an RDS between about 0.005 nm⁻¹ and about 0.06 nm⁻¹.
 8. The fiber stretcher module of claim 7, wherein the optical fiber stretcher module in the 1550 nm range comprises a RDS of about 0.042 nm⁻¹, and a RDC of about 0.00295 nm⁻¹. 